I want to know this, because I deleted some ncf/gcf files for portal 2, because the updating was stuck. And now it wants to download it all (11GB) again, so it would be nice if I could copy the content to the download location, without Steam noticing.

Aren't you worried about file corruption that might cause Steam to choke again, resulting in all 11 GB having to be redownloaded anyway?
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BoltClockJul 6 '11 at 2:36

@The Annoying Pyro: Even if some files are corrupted, Steam won't have to download all of them. The new format (NCF) which is used for all newer games including Portal 2 is able to download indiviual files instead of whole packs (GCF).
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KoraktorJul 6 '11 at 7:51